The human proteasome may be a target for the treatment of cancers such as multiple myeloma (MM) and relapsed/refractory mantle cell lymphoma (MCL). Current therapeutics for these diseases include compounds that target the proteasomes in the cancerous cells. However, the available therapeutic compounds are competitive inhibitors that bind through a covalent and irreversible (or slowly reversible) bond to the N-terminal threonine of the enzyme's catalytic sites. And unfortunately, more than 97% of multiple myeloma patients develop resistance or become intolerant to the currently available competitive inhibitors within a few years, after which survival is often less than one year. Further, examples of small molecules that act as non-competitive proteasome inhibitors are very scarce and exhibit activity only at high micromolar concentrations or non-physiologically relevant concentrations.